Rename "tables" concept

**Is your feature request related to a problem? Please describe.**

We've got ValidatorTable and `tables` in the config, but they're not really tables in the case of orc or parquet files. Let's get rid of the tables moniker and choose something else.

**Describe the solution you'd like**

ValidatorDataSource and `sources` might be more appropriate.

N.b. this would be a breaking change.
